Pre-converter for methanol synthesis

ABSTRACT

A reactor system, plant and a process for the production of methanol from synthesis gas is described in which the reactor system comprises:  
     (a) a first reactor adapted to be maintained under methanol synthesis conditions having inlet means for supply of synthesis gas and outlet means for recovery of a first methanol-containing stream, said first reactor being charged with a first volume of a methanol synthesis catalyst through which the synthesis gas flows and on which in use, partial conversion of the synthesis gas to a product gas mixture comprising methanol and un-reacted synthesis gas will occur adiabatically; and  
     (b) a second reactor adapted to be maintained under methanol synthesis conditions having inlet means for supply of the gaseous first methanol-containing stream, outlet means for recovery of a second methanol-containing stream and cooling means, said second reactor being charged with a second volume of a methanol synthesis catalyst through which the gaseous first methanol-containing stream flows and on which, in use, further conversion of the synthesis gas to a product gas mixture comprising methanol will occur.

[0001] The present invention relates to a process, reactor system andplant for the production of methanol. In particular, it relates to aprocess, reactor system and plant for producing methanol from hydrogenand carbon oxides.

[0002] Methanol is synthesised in large volumes annually by theconversion of a carbonaceous feedstock, such as natural gas, into amixture of carbon oxides and hydrogen. Such a mixture of gases in oftenreferred to as synthesis gas.

[0003] The conversion of a hydrocarbon-containing feedstock, such asnatural gas, into synthesis gas can be achieved by steam reforming, bypartial oxidation, or by a combination of these processes.

[0004] In steam reforming a mixture of desulphurised hydrocarbonfeedstock, such as natural gas, and steam is passed at high temperature,typically at a temperature of from about 600° C. to about 1000° C., andelevated pressure, typically from about 10 bar up to about 50 bar, overa suitable reforming catalyst, such as a supported nickel catalyst. Onecommercially recommended catalyst which is suitable for this purposeuses a mixture of calcium and aluminium oxides as support for thenickel. The principal reaction is:

CH₄+H₂O≈CO+H₂.

[0005] The reaction products themselves are further subject to thereversible “water gas shift” reaction in which carbon dioxide andhydrogen are produced from carbon monoxide and steam:

CO+H₂O≈CO₂+H₂.

[0006] Another method for producing synthesis gas involves the use,wholly or in part, depending upon the carbon to hydrogen ratio in thehydrocarbonaceous feedstock, of direct catalytic or non-catalyticpartial oxidation or secondary/autothermal reforming with oxygen. In thecase of methane this occurs according to the following equation:

CH₄+½O₂≈CO+H₂.

[0007] A combination of steam reforming and partial oxidation orsecondary/autothermal reforming can also be used.

[0008] Conversion of the carbon oxides and hydrogen to methanol occursaccording to the following reactions:

CO+2H₂≈CH₃OH

CO₂+3H₂≈CH₃OH+H₂O.

[0009] These reactions are conventionally carried out by contacting thesynthesis gas with a suitable methanol synthesis catalyst under anelevated synthesis gas pressure, typically in the range of from about 50bar up to about 100 bar, usually about 80 bar, and at an elevatedmethanol synthesis temperature, typically from about 210° C. to about270° C. or higher, e.g. up to about 300° C.

[0010] Suitable methanol synthesis catalysts include copper containingcatalysts with a catalyst comprising a reduced zinc oxide/copper oxidemixture being particularly-suitable.

[0011] As with many reactions it is desirable to achieve the maximumrate of reaction per weight of catalyst or per volume of the reactor.

[0012] A conventional methanol synthesis plant can be considered tocomprise four distinct parts, namely:

[0013] 1. a reforming plant, which produces a mixture of carbon oxidesand hydrogen from a hydrocarbon feedstock;

[0014] 2. a compression stage which lifts the carbon oxides and hydrogenmixture to a higher pressure suitable for downstream methanol synthesis;

[0015] 3. a methanol synthesis section, in which crude methanol isproduced from the carbon oxides and hydrogen; and

[0016] 4. a distillation section, in which the final refined methanolproduct is produced from the crude methanol.

[0017] A number of different types of reformer for use in part 1 of themethanol synthesis plant i.e. the reforming plant, are known in the art.One such type is known as a “compact reformer” and is described inWO-A-94/29013, which is incorporated herein by reference and whichdiscloses a compact endothermic reaction apparatus in which a pluralityof metallic reaction tubes are close-packed inside a reformer vessel.Fuel is burned inside the vessel, which comprises air and fueldistribution means to avoid excessive localised heating of the reactiontubes. In a compact reformer of this type heat is transferred from theflow gas vent and from the reformed gas vent of the reformer to incomingfeedstock, fuel and combustion air. Other types of reformer are not asefficient as the compact reformer in transferring heat internally inthis way. However, many other reformer designs are known and some aredescribed in EP-A-0033128, U.S. Pat. No. 3,531,263, U.S. Pat. No.3,215,502, U.S. Pat. No. 3,909,299, U.S. Pat. No. 4,098,588, U.S. Pat.No. 4,692,306, U.S. Pat. No. 4,861,348, U.S. Pat. No. 4,849,187, U.S.Pat. No. 49,090,808, U.S. Pat. No. 4,423,022, U.S. Pat. No. 5,106,590and U.S. Pat. No. 5,264,008, U.S. Pat. No. 5,264,008 and WO 98/28071which are incorporated herein by reference.

[0018] In a conventional plant, synthesis gas is compressed in passagefrom the reforming plant to the methanol synthesis zone. Thiscompression stage is generally present in order to provide the requiredpressure of from 50 bar to 100 bar in the methanol synthesis zone. Thecompressed gas is then passed to the methanol synthesis section.

[0019] In U.S. Pat. No. 4,594,227 apparatus for carrying out a catalyticchemical reaction is described which comprises a vertical, annular,intercylinder space which is divided by radially extending verticalpartition walls into a plurality of chambers some of which includeheat-exchanging tubes. Segments containing no heat-exchanging tubes maybe packed with catalyst and utilised adiabatically to preheat thereaction gases. In use, the reaction gases will pass outwardly throughthis first segment, where any reaction will cause heating, they then aretransmitted through the annular space surrounding the intercylinderspace before travelling inwardly through the segment containing catalystand cooling tubes where further reaction will occur.

[0020] Whilst this arrangement does offer certain advantages, it alsosuffers from various drawbacks. A principle disadvantage arises from themulti-segmental radial flow. This flow pattern causes the gas velocityto vary as the flow traverses from the centre of the reactor to theoutside and back, due to the changing cross-sectional area of thesegments. This changing velocities of the segments causes the heattransfer coefficient between the reacting gases and the cooling mediumin the tubes to vary. In particular the heat transfer will increase asthe gas velocity increases and will decrease as the gas velocity isreduced.

[0021] Thus the multi-segmental arrangement of the radial flow apparatusin U.S. Pat. No. 4,594,227 does not allow the gas velocity pattern andresultant heat transfer pattern to be optimised.

[0022] Various methanol production processes are known in the art, andreference may be made, for example, to U.S. Pat. No. 5,610,202, U.S.Pat. No. 4,968,722, U.S. Pat. No. 5,472,986, U.S. Pat. No. 4,181,675,U.S. Pat. No. 5,063,250, U.S. Pat. No. 4,529,738, U.S. Pat. No.4,595,701, U.S. Pat. No. 5,063,250, U.S. Pat. No. 5,523,326, U.S. Pat.No. 3,186,145, U.S. Pat. No. 344,002, U.S. Pat. No. 3,598,527, U.S. Pat.No. 3,940,428, U.S. Pat. No. 3,950,369, WO-A-98/28248 and U.S. Pat. No.4,051,300 which are incorporated herein by reference.

[0023] Various suggestions have been made for modifications to the plantdesign with a view to improving the economics of the production process.

[0024] Several suggestions for improving the efficiency of the reactionhave been made which incorporate the use of multiple reaction stages.For example, in U.S. Pat. No. 5,631,302 it is suggested that themethanol synthesis section should include two separate synthesisreactors. In this arrangement, the synthesis gas is passed to the firstsynthesis reactor, which is a shaft reactor containing a fixed bed of acopper-containing catalyst. The reaction in this shaft reactor iscarried out adiabatically and in the absence of any recycling ofsynthesis gas. The product stream from this first reactor, whichcontains methanol vapour, is cooled to condense the methanol which isseparated from the unreacted gaseous. components of the first productstream. These unreacted gaseous components are then compressed, heatedand fed to the second reactor where they react to form methanol. Thesecond reactor is preferably a tubular reactor in which the coppercatalyst is indirectly cooled by water which is boiling under highpressure. The product stream from the second reactor is cooled and themethanol is removed by separation. Any unreacted gaseous components arecompressed and heated before being returned to the second reactor.

[0025] Thus in U.S. Pat. No. 5,631,302 the first reactor is locatedoutside the main reactor loop and simply serves to modify thecomposition of the feed gas before it enters the main reaction loop. Thearrangement of U.S. Pat. No. 5,631,302 is said to be useful where thesynthesis gas feed has a CO₂:CO ratio which exceeds 2:1.

[0026] An alternative arrangement is suggested in U.S. Pat. No.5,827,901. In this arrangement two synthesis reactors are connected inseries such that the product stream from the first reactor is passeddirectly to the inlet of the second reactor. The first reactor is awater cooled reactor in which the catalyst is located in tubes throughwhich the gaseous reactants flow. The second reactor may be selectedfrom a variety of designs. Whichever design is used, cooling in thesecond reactor is provided by counter-current heat exchange with thefeed synthesis gas before it is fed to the first reactor.

[0027] This arrangement allows for cooler exit temperatures from thesecond reactor to be achieved than are conventionally achievable.However, whilst the lower temperature may allow the reaction equilibriumto move towards completion, it may also reduce the rate of reaction, andtherefore may require more catalyst per unit of product.

[0028] Other examples include U.S. Pat. No. 5,427,760 in which tworeaction stages are used in an attempt to achieve a higher overallconversion to the desired ammonia than can be achieved in a single stageand U.S. Pat. No. 4,867,959 in which two or more reaction stages aredescribed, with cooling between each stage, to increase conversion. Asdiscussed by Kobayashi and Green in a paper presented to the 1990 WorldMethanol Conference, this approach can be extended to include a largenumber of stages. This paper also illustrates the optimum rate line formethanol synthesis.

[0029] Whilst an optimum rate line is known, a near-optimum reactionprofile is not practical in commercial arrangements. This is becausesuch a profile would generally require the reaction to start at hightemperature and gradually fall as the reaction proceeds. Somesuggestions have been made to produce a system which approaches theoptimum rate line, such as those in the Kobayashi and Green paperhowever, a commercially satisfactory arrangement has not been realised.

[0030] Thus it will be understood that whilst the systems of the priorart go some way to addressing the problems associated with reducing theoperating and/or investment costs of producing methanol, variousdisadvantages and drawbacks remain and there is still a requirement foralternative arrangements which will address at least some of theseproblems.

[0031] One alternative arrangement which goes at least some way toaddressing these problems is an arrangement in which the catalyst forthe production of methanol in the methanol synthesis section of theplant is divided into two volumes of independent geometry. Anarrangement of this type will allow the highest temperature to which thecatalyst is subjected to be reduced whilst also reducing the overallreactor pressure drop.

[0032] Thus, according to a first aspect of the present invention thereis provided a reactor system for use in the production of methanol fromsynthesis gas comprising:

[0033] (a) a first reactor adapted to be maintained under methanolsynthesis conditions having inlet means for supply of synthesis gas andoutlet means for recovery of a first methanol-containing stream, saidfirst reactor being charged with a first volume of a methanol synthesiscatalyst through which the synthesis gas flows and on which in use,partial conversion of the synthesis gas to a product gas mixturecomprising methanol and unreacted synthesis gas will occuradiabatically; and

[0034] (b) a second reactor adapted to be maintained under methanolsynthesis conditions having inlet means for supply of the gaseous firstmethanol-containing stream, outlet means for recovery of a secondmethanol-containing stream and cooling means, said second reactor beingcharged with a second volume of a methanol synthesis catalyst throughwhich the gaseous first methanol-containing stream flows outwardly fromthe inlet means and on which, in use, further conversion of thesynthesis gas to a product gas mixture comprising methanol will occur.In one preferred arrangement the cooling means is arranged such thatheat transfer decreases as the gas flows from the inlet to the outlet.

[0035] Thus the present invention provides an arrangement in which anadiabatic bed is combined with a downstream cooled catalyst bed. Thisarrangement allows a commercially acceptable process to be providedwhich, in the most preferred arrangement has a reaction profile whichwill approximately follow the optimum rate line.

[0036] In a preferred arrangement, the first and second reactors areseparate reactors and the outlet means of the first reactor is connectedto the inlet means of the second reactor by conventional means.

[0037] However, the first and second reactors may be zones locatedwithin a single reactor. In this latter arrangement it is important thatthe catalyst of the first and second reactors do not have a common faceand the reactor system will therefore preferably include means forseparating the two catalyst volumes and for transferring the gaseousfirst methanol-containing stream from the outlet means of the firstreactor to the inlet means of the second reactor. It will be understoodthat in this arrangement the “outlet means” and “inlet means” may beareas of the reactor rather than specific items of construction. The oreach reactor is preferably a pressure vessel.

[0038] The first volume of the methanol synthesis catalyst is preferablyarranged as a horizontal volume and the apparatus is preferably arrangedsuch that the synthesis gas preferably flows through the catalyst volumein a vertical direction. In a particularly preferred. arrangement, thedepth of the first volume of catalyst is preferably less than itshorizontal dimensions. Thus, where the first volume of catalyst iscylindrical, the depth of the cylinder is less than the diameter of thecross-section of the cylinder or where it is a prism, the depth will beless than the cross-sectional dimensions.

[0039] One benefit associated with reducing the depth of the bed is thatthe pressure drop of the gases as they pass through the bed is reduced.This has the effect of reducing the overall cost of the plant. A furtheradvantage of the arrangement of the present invention is that theincreased cross-sectional area of the catalyst in the first volumepresented to the synthesis gas relative to the depth of the first volumeof catalyst through which the gas has to flow when compared to prior artcatalyst volumes in vertical vessels facilitates the heat transfer byconduction and/or radiation from the relatively hotter catalyst at thebottom of the bed to the relatively cooler catalyst located towards theupper surface of the bed. The resultant increase in the averagetemperature of the catalyst bed serves to increase the rate of reaction.

[0040] Whilst the horizontal arrangement of the catalyst volume ispreferred, any suitable arrangement may be utilised provided that it isof a low pressure drop design. In one alternative arrangement the firstvolume of catalyst in the first reactor allows for radial flow.

[0041] The first volume of catalyst will be retained in position withinthe reactor by any suitable means and is preferably located on a supportmeans, such as a grid, which allows the gaseous reactants to passthrough the catalyst volume with minimal reduction in gas pressure. Thefirst catalyst volume is preferably a fixed bed arrangement.

[0042] The first reactor may additionally include an inlet gasdistributor to assist in achieving good distribution of the synthesisgas throughout an upper area of the reactor before the gas comes intocontact with the catalyst volume.

[0043] Any suitable arrangement for the second reactor may be used.Further.-any suitable arrangement for the second volume of catalyst inthe second reactor may be used. In a preferred arrangement, the reactoris designed for a minimum pressure drop. In a most preferredarrangement, the second volume of catalyst in addition to providingminimum pressure drop is also arranged to allow good heat transfer fromthe catalyst to a cooling means. The presence of the cooling means isparticularly preferred as it provides that the exit temperature of thegas is reduced towards the equilibrium value and is prevented fromrising significantly which would result in faster rates of catalystdeactivation.

[0044] In a particularly preferred arrangement, the product stream fromthe first reactor flows radially from a central inlet to an outletcollector located at a reactor zone wall through the second catalystvolume.

[0045] The catalyst may be cooled by any suitable means. In onepreferred arrangement, cooling is provided by boiling water cooling intubes which pass through the catalyst bed in a conventional manner. Thismethod of cooling allows steam to be produced which may then be used todrive the compressor which may be present to increases the pressure ofthe feed or recycle synthesis gas prior to its addition to the firstreactor zone.

[0046] Producing the steam for use in driving the compressor in thismanner has various benefits. In particular, the overall efficiency ofthe plant may be improved which will reduce the overall costs.

[0047] The catalyst in the first and second volumes may be the same ordifferent. The catalyst for use in the methanol synthesis in eachreactor is preferably selected from, but is not limited to,copper-containing catalysts, for example reduced CuO—ZnO catalysts.Preferred catalysts include those sold under the designation 51/8 by ICIKatalco. Other suitable catalysts are described in U.S. Pat. No.6,054,497 which is incorporated herein by reference.

[0048] According to a second aspect of the present invention there isprovided a plant for the production of methanol from a synthesis gasmixture comprising carbon oxides, hydrogen and methane comprising:

[0049] (a) a methanol synthesis zone including the reactor systemaccording to the above-mentioned first aspect of the present invention;and

[0050] (b) a methanol recovery zone, adapted to be maintained undermethanol recovery conditions, for recovery of a crude methanol productstream from the product gas mixture, and for recovery of a vaporousstream comprising unreacted material of the synthesis gas.

[0051] In a preferred arrangement the plant additionally includes:

[0052] (c) means for recycling at least a portion of the unreactedmaterial of the synthesis gas from the methanol recovery system to themethanol synthesis zone.

[0053] The synthesis gas mixture is preferably produced from ahydrocarbon feedstock material in plant comprising a steam reformingzone, adapted to be maintained under steam reforming conditions andcharged with a catalyst effective for catalysis of at least one steamreforming reaction, for steam reforming of a vaporous mixture of thehydrocarbon feedstock in the steam to form a synthesis gas mixturecomprising carbon oxides, hydrogen and methane. Suitable steam reformersinclude those detailed above which are incorporated herein by reference.

[0054] The plant of the present invention may include a plurality ofreactor systems according to the above first aspect of the presentinvention. These may be located in parallel such that the overall plantproduction of methanol may be increased or in an alternativearrangement, they may be located in series such that the secondmethanol-containing stream is passed either directly or indirectly to afirst reactor zone of a second reactor system. One benefit of thisarrangement is that the recovery of reactants is improved.

[0055] According to a third aspect of the present invention there isprovided a process for producing methanol from a synthesis gascomprising:

[0056] (a) supplying the synthesis gas mixture to the methanol synthesisreactor system of the above-mentioned first aspect of the presentinvention maintained under methanol synthesis conditions;

[0057] (b) recovering from the methanol synthesis reactor system aproduct gas mixture comprising methanol and any unreacted material ofthe synthesis gas mixture;

[0058] (c) supplying material of the product gas mixture to a methanolrecovery zone maintained under methanol recovery conditions; and

[0059] (d) recovering from the methanol recovery zone a crude methanolproduct stream and a vaporous stream comprising unreacted material ofthe synthesis gas mixture.

[0060] In a preferred arrangement, the process additionally includes thestep of recycling the unreacted material of the synthesis gas mixture tothe methanol synthesis reactor.

[0061] The synthesis gas is preferably formed from a hydrocarbonfeedstock in a process comprising contacting a vaporous mixturecomprising the feedstock and steam in the steam reforming zone with acatalyst effective for catalysis of at least one reforming reaction andrecovering from the reforming zone a synthesis gas mixture comprisingcarbon oxide, hydrogen and methane.

[0062] The synthesis gas is preferably compressed before being suppliedto the methanol synthesis reactor system. The pressure of the gaseousreactants entering the first reactor zone will preferably in the regionof 20 bar to 200 bar. The first volume of catalyst in the first reactoris preferably arranged such that the gas pressure drop that occurs isless than 0.5 bar. The compression may occur by any suitable means.Typically the motive force of gas compression is provided by highpressure steam generated within the plant by a steam turbine. However,as has been discussed, the steam may be wholly or in part provided bythe cooling system in the second reactor.

[0063] The temperature of the gaseous reactants entering the firstreactor will preferably be in the region of about 180° C. to about 220°C. The reactants exiting the first reactor and entering the secondreactor will be substantially the same temperature. The temperature ofthese streams is most preferably just below peak reaction temperatureand will therefore most preferably be in the region of about 230° C. toabout 350° C.

[0064] The space velocity of the synthesis gas mixture entering thefirst volume of catalyst is preferably in the region of 5 to 20% of thetotal space velocity, dependent on the syntheses gas composition.

[0065] The apparatus, plant and process of the present invention havesignificant advantages over conventional apparatus, plant and processesfor the production of methanol.

[0066] Most significantly the invention allows the first volume ofcatalyst to be designed to give a low pressure drop, while the secondvolume of catalyst is designed to meet the additional requirement tocontrol reaction temperature. By separating the two volumes flexibilityis attained which leads to enhanced performance of the reactor in termsof reactor pressure drop, steam production pressure and reactorconversion.

[0067] One further benefit of the present invention is that thearrangement in the first reactor zone is simple to manufacture whichsubstantially reduces the cost of construction.

[0068] Whilst generally there is no economic benefit in dividing volumesof material in process plants, since this inevitably leads to anincrease in aspects of reactor construction cost per unit volume, in thepresent invention substantial benefits are obtained.

[0069] For example, by reducing the reaction system pressure drop, therequirement to compress the recycle gas stream is reduced and hence thecost of recycle gas compression is reduced. A slight increase inconversion of synthesis gas to methanol may also be noted. However, moreimportantly, the improved efficiency of the present invention may meanthat the total catalyst volume required for a given methanol productionrate is reduced.

[0070] The system has the further benefit in that the maximumtemperature achieved in the reaction zones is reduced which will reducethe rate at which catalyst deactivation occurs.

[0071] The present invention will now be described, by way of example,with reference to the accompanying drawings in which:

[0072]FIG. 1 is a representation of a reactor system in accordance withthe present invention;

[0073]FIG. 2 is a schematic diagram of a process in accordance with thepresent invention;

[0074]FIG. 3 is a representation of a reactor system according to thecomparative example; and

[0075]FIG. 4 is a graph comparing results obtained in reactions in thesystem of FIGS. 1 and 3.

[0076] It will be understood by those skilled in the art that thedrawings are diagrammatic and that further items of equipment such asfeedstock drums, pumps, vacuum pumps, compressors, gas recyclingcompressors, temperature sensors, pressure sensors, pressure reliefvalves, control valves, flow controllers, level controllers, holdingtanks, storage tanks and the like may be required in a commercial plant.Provision of such ancillary equipment forms no part of the presentinvention and is in accordance with conventional chemical engineeringpractice.

[0077] Referring to FIG. 1, the methanol reaction system of the presentinvention comprises a first reactor vessel 1 having a gas inlet andthrough which in use synthesis gas at a temperature of 180° C. to 220°C. and a pressure of 20 bar to 200 bar enters. The gas is preferablyevenly distributed within the upper area 3 of the vessel 1 by means ofan inlet gas distributor 4.

[0078] The synthesis gas then passes through a first volume of catalystcomprising a bed of a suitable methanol synthesis catalyst 5 supportedon a grid 6. The catalyst is preferably a copper-containing catalystsuch as CuO—ZnO. The bed of catalyst 5 has a high cross-section to depthratio when compared with conventional systems. The depth of the bed ispreferably between about 0.4 to about 1.2 metres. This low depth to thecatalyst bed reduces the pressure drop of the gaseous mixture as itpasses through the bed.

[0079] Some of the synthesis gas will undergo conversion adiabaticallyto methanol as it is passed through the catalyst bed 5 such that thegases collected by the gas collector 7 include un-reacted synthesis gasand methanol. This then leaves reactor 1 via outlet 8, and the gaseousstream is passed in line 9 to the inlet 10 to the second reactor zone11.

[0080] This second reactor may be of any suitable design but preferablyhas a central gas distributor 12 and is constructed to allow for radialgas flow from the central gas distributor 12 to the outlet collector 13at the vessel wall of reactor 11. Thus gas flow is predominantly radialsuch that the gaseous mixture will pass through the catalyst bed 14.

[0081] The bed 14 is cooled by water boiling in a plurality of tubes 33which pass through the catalyst bed between tube sheets 34 located aboveand below-the catalyst bed. Pressurised cooling water is introduced viainlet nozzle 35 and steam and water exits via the exit nozzle 36. Thereactor also includes a utility nozzle 37.

[0082]FIG. 2 illustrates the flow sheet of the process of the presentinvention in which synthesis gas, which may have been formed in a steamreforming plant (not shown) is fed at a pressure of from 20 bar to 200bar at a temperature of from about 180° C. to about 220° C. to the firstreactor volume 1 in stream 38 where reaction occurs adiabatically.

[0083] The first product stream which will be at a pressure from about20 bar to about 200 bar and a temperature of from about 200° C. to about250° C. and which includes un-reacted synthesis gas and methanol ispassed to the second reactor volume 11 where further reaction occurs.The product stream 39 collected from the second reactor 11 is passed tothe cooling heat exchanger train 40 where the methanol is condensed.Stream 41 is therefore of mixed phase including un-reacted synthesis gasand condensed methanol. These are separated in separator 42 and a crudemethanol product stream is retrieved in line 43 for further purificationby conventional means such as distillation.

[0084] The gas stream 44 from the separator will generally be dividedinto a recycle stream 45 and a purge gas stream 51. The purge gas streamwill remove inert materials and optionally excess hydrogen.

[0085] Gas stream 45 is then compressed in gas compressor 46 to asufficient pressure to allow recycling to the first reactor zone 1. Theresultant stream 47 may be combined with fresh synthesis gas via line48. In one alternative arrangement, the fresh synthesis gas may be addedinto line 45, i.e. before the recycle stream is passed through thecompressor.

[0086] The resultant stream will then pass through heat exchangers 49before being passed in stream 50 to the first reactor zone 1.

[0087] In one arrangement the heat exchangers 40 and 49 may be combinedsuch that the hot product stream 39 is cooled in counter-current heatexchange with the recycle stream 47 which is consequently warmed.

[0088]FIG. 3 illustrate an reactor of generally conventional designwhich will produce the same quantity of methanol from the same gas feedas that illustrated in FIG. 1. In this arrangement the conventionaldesign has been slightly modified to more closely be comparable to thearrangement of FIG. 1 and thus on an inner part of the catalyst volume52 has no cooling tubes passing through and therefore an essentiallyadiabatic bed is achieved through which the synthesis gas flows afterentry through inlet 54.

[0089] As the gas flow is radial towards outlet 55, after passingthrough the inner volume, the gas passes through an other part 53 of thecatalyst volume through which cooling tubes 56 pass. Thus the secondpart of the volume functionally corresponds with the second reactor ofFIG. 1. However, in order to achieve the same production rate whilemaintaining the same vessel diameter, the height of the vessel has to beincreased over that required in the arrangement of FIG. 1. This ispartly due to the inclusion of the adiabatic volume of catalyst.However, it is also due to reduced gas velocities caused by theincreased height over the increased mean diameter of the cooled catalystvolume which causes a lower heat transfer coefficient. Thus to removethe same amount of heat without increasing the peak catalysttemperature, a larger surface area and hence a large volume of coolingtubes has to be provided. The lower gas velocities also result in poorerheat transfer and thus poorer temperature control within the cooled bed.

[0090] Further an increased volume of catalyst is required since theexit temperature of the gas from the catalyst bed is reduced whichreduces the rate of reaction per unit volume of catalyst. Thus, thisarrangement would be wholly uneconomical.

EXAMPLE 1 AND COMPARATIVE EXAMPLE 1

[0091] Reactive systems in accordance with those illustrated in FIG. 1and FIG. 3 are fed with a feed gas having the following composition:Component Volume % Steam 0.4 Hydrogen 72.5 Carbon Monoxide 15.4 CarbonDioxide 7.3 Methane 3.3 Nitrogen 1.1 Total 100.0

[0092] The results obtained are as follows: Comparative Example 1Example 1 Catalyst Volume, relative 1.0 1.1 Methanol Production Per Unit1.0 0.9 Volume (relative) Generated Steam pressure bar 25.5 25.5 ReactorPressure Drop bar 1.4 1.15 Heat Transfer surface, relative 1.0 1.09Inlet gas temperature ° C. 221 218

[0093] This performance data is for new catalyst which has been un-agedby plant operation. The distribution of catalyst volume versestemperature within the reactors is illustrated in FIG. 4. As can beseen, from the arrangement of the present invention of Example 1 has 49%of the catalyst operating above a temperature of 250° C. In contrast, inthe single volume arrangement of Comparative Example 1 and FIG. 3, thetotal amount of catalyst is increased to 110% of that of Example 1 and60% of this catalyst is operating above 250° C.

[0094] Thus it would be understood that the arrangement of the presentinvention allows the amount the catalyst required to be reduced, for animproved performance with the same amount of the catalyst, and for thecatalyst to be operated at lower temperatures which will prolongcatalyst life.

[0095] Whilst the present invention has been discussed with regard tothe production of methanol from synthesis gas; it will be understoodthat the reactor design of the present invention may also be applied toother exothermic chemical reactions, such as the formation of ammonia.

1. A reactor system for use in the production of methanol from synthesisgas comprising: (a) a first reactor adapted to be maintained undermethanol synthesis conditions having inlet means for supply of synthesisgas and outlet means for recovery of a first methanol-containing stream,said first reactor being charged with a first volume of a methanolsynthesis catalyst through which the synthesis gas flows and on which inuse, partial conversion of the synthesis gas to a product gas mixturecomprising methanol and un-reacted synthesis gas will occuradiabatically; and (b) a second reactor adapted to be maintained undermethanol synthesis conditions having inlet means for supply of thegaseous first methanol-containing stream, outlet means for recovery of asecond methanol-containing stream and cooling means, said second reactorbeing charged with a second volume of a methanol synthesis catalystthrough which the gaseous first methanol-containing stream flowsoutwardly from the inlet means and on which, in use, further conversionof the synthesis gas to a product gas mixture comprising methanol willoccur.
 2. A reactor system according to claim 1 wherein the first andsecond reactors are located in separate reactors and the outlet means ofthe first reactor is connected to the inlet of the second reactor.
 3. Areactor system according to claim 1 wherein the first and secondreactors are located within a single reactor.
 4. The reactor systemaccording to any one of claims 1 to 3 wherein the catalyst volume of thefirst reactor is arranged as a horizontal volume.
 5. The reactor systemaccording to any one of claims 1 to 4 wherein the depth of the firstcatalyst volume is preferably less than its horizontal dimensions. 6.The reactor system according to any one of claims 1 to 5 wherein thefirst reactor additionally includes an inlet gas distributor.
 7. Thereactor system according to any one of claims 1 to 6 wherein the secondvolume of catalyst is arranged such that the product stream from thefirst reactor flows radially from a central inlet through the secondcatalyst volume to an outlet collector located at a reactor wall.
 8. Thereactor system according to any one of claims 1 to 7 wherein the coolingof the catalyst in the second reactor is provided by water cooling intubes which pass through the catalyst bed.
 9. The reactor systemaccording to any one of claims 1 to 8 additionally including apparatusto enable the steam produced by the water cooling to be used to drive acompressor which may be present to increases the pressure of the feed orrecycle synthesis gas prior to its addition to the first reactor.
 10. Aplant for the production of methanol from a synthesis gas mixturecomprising carbon oxides, hydrogen and methane comprising: (a) amethanol synthesis zone including the reactor system according to anyone of claims 1 to 9; and (b) a methanol recovery zone, adapted to bemaintained under methanol recovery conditions, for recovery of a crudemethanol product stream from the product gas mixture, and for recoveryof a vaporous stream comprising un-reacted material of the synthesisgas.
 11. The plant according to claim 10 additionally including: (c)means for recycling at least a portion of the un-reacted material of thesynthesis gas to the methanol synthesis zone.
 12. The plant according toclaim 10 or claim 11 comprising a plurality of reactor systems accordingto any one of claims 1 to
 9. 13. The plant according to claim 12 whereinthe plurality of reactor systems are located in parallel or in series.14. The plant according to any one of claims 10 to 13 wherein thesynthesis gas mixture is produced from a hydrocarbon feedstock materialin plant comprising a steam reforming zone, adapted to be maintainedunder steam reforming conditions and charged with a catalyst effectivecatalysis of at least one steam reforming reaction, for steam reformingof a vaporous mixture of the hydrocarbon feedstock in the steam to forma synthesis gas mixture comprising carbon oxides, hydrogen and methane.15. A process for producing methanol from a synthesis gas comprising:(a) supplying the synthesis gas mixture to the methanol synthesisreactor system of any one of claims 1 to 12 maintained under methanolsynthesis conditions; (b) recovering from the methanol synthesis reactorsystem a product gas mixture comprising methanol and an un-reactedmaterial of the synthesis gas mixture; (c) supplying material of theproduct gas mixture to a methanol recovery zone maintained undermethanol recovery conditions; and (d) recovering from the methanolrecovery zone a crude methanol product stream and a vaporous streamcomprising un-reacted material of the synthesis gas mixture.
 16. Aprocess according to claim 15 additionally including the step ofrecycling the un-reacted material to the methanol synthesis reactor. 17.The process according to claim 15 or claim 16 wherein the synthesis gasis formed from a hydrocarbon feedstock in a process comprisingcontacting a vaporous mixture comprising the feedstock and steam in thesteam reforming zone with a catalyst effective for catalysis of at leastone reforming reaction and recovering from the reforming zone asynthesis gas mixture comprising carbon oxide, hydrogen and methane. 18.The process according to any one of claims 15 to 17 wherein thesynthesis gas is compressed before being supplied to the methanolsynthesis reactor system.
 19. The process according to any one of claims15 to 18 wherein the pressure of the gaseous reactants entering thefirst reactor zone are in the region of 20 bar to 200 bar.
 20. Theprocess according to any one of claims 15 to 19 in which the motiveforce of gas compression is provided by high pressure steam generatedwithin the plant by a steam turbine.
 21. The process according to anyone of claims 15 to 19 in which the motive force for gas compression iswholly or in part provided by the cooling system in the second reactorzone.
 22. The process according to any one of claims 15 to 19 whereinthe temperature of the gaseous reactants entering the first reactor zoneare in the region of 180° C. to 220° C.